I found this post on Revit City and am having similar problems.
"We are running into some issue when trying to import images into Revit. Problem is that Revit seems make the default resolution for the jpeg as 72 dpi, and adjusts the size of the image to whatever in a conversion process to account for the images import resolution. This is a significant and seemingly unnecessary pain. We have a lot of images and scans that we incorporate into our drawings for presentation and otherwise that need to be at a lot higher resolution than 72 dpi. Is there any kind of preference or global control for this that can be adjusted? How are other folks handling this problem. Do folks just save their drawings at or 288 dpi and scale accordingly in Revit? Thanks in advance for your assistance."